1. Field of the Invention
This invention relates to impact sockets, and in particular to lightweight sockets such as extended length impact sockets.
2. Background of the Invention
Impact sockets are well known in the art. Impact sockets are rotated by impact wrenches, impact guns or guns. These guns are designed to deliver high torque output with minimum exertion by the user, by storing energy in rotating mass such as a hammer, and the sudden delivery of the energy to the output shaft. Guns use compressed air, hydraulics or electrical energy. Guns are used for precise output torque. Impact sockets can be used for every standard socket, wrench drive size, from small one quarter inch sizes to over three and one half inch sizes.
Impact sockets are traditionally made from high strength alloy steel. Steel is a particularly good material for impact sockets because it is able to absorb considerable energy without failure. The ability of a metal to absorb energy without fracture is known as “toughness.” “Fracture toughness” is a measure of the stress required to propagate cracking in a material that contains microflaws such as grain boundaries and second phase particles or occlusions. Steel sockets are forged to achieve grain orientation in the material that gives the socket high fracture toughness when loaded. Type 4047 steel is often used in impact sockets. Other types of high strength alloy steel are Type 4140 and Type 4340.
One problem with steel is its relative heaviness when compared with lightweight metals such as aluminum. Lightweight impact sockets are much desired when used overhead, particularly with extended length impact sockets such as those sockets that are greater than 3½ inches in length. In one instance, a lightweight deep impact socket (a deep socket is used for turning a nut on a bolt, and the bolt extends into the socket) was made from aluminum. Each impact socket was made from 7075 aluminum, and the maker of the sockets was Fastorq Bolting Systems of Houston, Tex. They were turned by a Racine hydraulic impact gun, and also by a Fairmount hydraulic impact gun. The impact guns were listed as being capable of producing 500 foot-pounds torque. Four 7075 aluminum sockets were tested, and all failed. The characteristics of the four failed aluminum impact sockets were as follows:
SizeWeightType of Failure1⅝ inch hex head1.56 lbs.Split half way down the lengthfrom the drive end.1⅝ inch hex head1.55 lbs.Fractured initiated in onecorner of the opening for thesquare ¾ inch drive.1 15/16 inch square head1.86 lbs.Incipient cracks in all fourcorners of the opening for thesquare ¾ inch drive.1½ inch hex head1.51 lbs.Cracks originated in three ofthe four corners of the openingfor the square ¾ inch drive.The 7075 aluminum alloy used for the four preceding aluminum impact sockets is the highest strength aluminum in the aluminum family. The 4047 steel is 2.8 times heavier than the 7075 aluminum, but the 4047 steel is far greater than 2.8 times resistant to breakage when impact loaded. Material strength properties vary with the speed of load application. An important difference between static and impact loading is that statically loaded sockets must be designed to carry loads, whereas impact sockets must be designed to absorb energy. Steel such as 4047 steel has the requisite toughness and fracture toughness. The aluminum impact sockets are machined, not forged, and have poor fracture toughness properties when loaded in torsion as the multiaxial stress state induced imposes simultaneous elastic and plastic deformation in the material. When torque is applied in the area of the drive stem, the material is not uniformly loaded across its load bearing cross section. Therefore, it is likely that the 7075 aluminum impact socket will develop cracks and fail no matter how thick and beefy it is made. An impact socket must be able to withstand wear, and 7075 aluminum has extremely poor wear resistant properties compared to 4047 heat-treated steel.